In general, the hydraulic systems for hydraulic excavators or other working machines include, as described in Patent Document 1, a hydraulic pump, multiple kinds of hydraulic actuators driven by a hydraulic fluid delivered from the hydraulic pump, a plurality of flow/directional control valves of the center bypass type that control a flow of the hydraulic fluid supplied from the hydraulic pump to the hydraulic actuators, a plurality of operating means provided for the hydraulic actuators in order to operate the respective flow/directional control valves, and a pump regulator that controls a capacity of the hydraulic pump such that a delivery rate of the hydraulic fluid therefrom changes in response to the operation of the multiple operating means.
Some of the hydraulic systems for hydraulic excavators or other working machines also include center bypass cutoff valves arranged for various purposes in center bypass lines passing through the center bypass type flow/directional control valves. Patent Document 2 introduces one such example. The center bypass cutoff valve in this example is disposed at a most downstream end of the center bypass line so that when the flow/directional control valve is switched to a boom lowering position by control lever operations while a boom angle to a swing structure stays in a predetermined range from a maximum angle, the center bypass cutoff valve is closed and the hydraulic fluid delivered from the hydraulic pump is forcibly supplied to a rod side of the boom cylinder. This ensures that even when the hydraulic excavator is disposed on a slope, the boom can be reliably moved downward. In addition, the flow/directional control valve for the boom contains a recovery circuit, and when the boom is in a state that it can move downward under its own weight, the hydraulic fluid delivered from a bottom side of the boom cylinder, in addition to the delivered hydraulic fluid from the hydraulic pump, can be supplied to the rod side of the boom cylinder via the recovery circuit. This speeds up the starting operation of downward movement of the boom while suppressing hydraulic pump energy consumption.